This invention relates to high purity refractories and, more particularly, it concerns an improved high purity fused grain made of alumina, chromia and magnesia oxides.
Commercial high purity refractories based upon alumina, chromia, and magnesia, are divided into two groups. One group is based upon picrochromite (MgO.multidot.Cr.sub.2 O.sub.3 in amounts of about 80% Cr.sub.2 O.sub.3 and 20% MgO) and the other group is based upon alumina-chromia in various combinations. Grains used to make these refractories are either sintered or fused. Fused grains are preferred due to their low porosity as it is very difficult to sinter high purity inert refractory oxides. Refractories based upon picrochromite are used in high temperature basic environments such as in critical wear areas of some steelmaking furnaces. Refractories based on alumina-chromia and chromia-alumina are used in high temperature more acidic environments such as coal gasifiers.
Many steelmaking or refining vessels have acidic slags initially. Later during the production cycle slag conditions turn basic. A classic example of this is the Argon Oxygen Decarburization (AOD) vessel. A desirable refractory for such applications would contain combinations of all three oxides--magnesia, alumina, and chromia. Such a refractory should have improved resistance to changing slag conditions compared to either end member (MgO-Cr.sub.2 O.sub.3 or A.sub.2 O.sub.3 -Cr.sub.2 O.sub.3). Such a refractory should also have improved thermal shock resistance compared to chrome-alumina compositions. A refractory based upon A.sub.2 O.sub.3 -Cr.sub.2 O.sub.3 -MgO should be a cost effective means for resisting a variety of slags within a given operation. The three component system should allow a decrease in the total chromic oxide level without negatively impacting slag resistance. Lowering the Cr.sub.2 O.sub.3 content of the refractory could significantly lower cost as chromic oxide is by far the most costly ingredient in this system.
U.S. Pat. No. 3,615,775 discloses compositions made from 80 to 95 wt. % alumina and the balance magnesia and chromia. This mixture when fired contained coarse alumina grains having some chromia in solid solution bonded by a MgO-Al .sub.2 O.sub.3 -Cr.sub.2 O.sub.3 matrix. U.S. Pat. No. 4,490,474 discloses a fused cast refractory consisting of a uniform mixture of 65 to 98 wt. % Cr.sub.2 O.sub.3, 0.75 to 4 wt. % MgO, and 1 to 34.25 wt. % Al .sub.2 O.sub.3. In both of these references, the magnesia content is so low that the beneficial aspects of a refractory containing higher levels of magnesia could not be realized.
In light of the foregoing, there is a need for an improved high purity refractory which is resistant to both acidic and basic metallurgical slags.